I want to redirect the execv of ls in a file and then in a pipe, why is not working?

Question

First of all, I want to redirect the output of ls (exec) in a file and then from a file to pipe, why is not working? It's ok when I redirect in a file, but that's all.

How can I do to find the length of the output of ls? (that's why I did a redirect to a file).

#include<stdlib.h>
#include<stdio.h>
#include<unistd.h>
#include<errno.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<stdlib.h>
#include<string.h>
#include<fcntl.h>

int main()
{
    char input[]="ls",ch, delim[]=" ",*result,aux[2000],*user=NULL,*password=NULL,aux1[2000],aux2[2000],aux3[2000],aux4[2000],*arg[1000];
    int p1[2],p2[2],i,j,len,nrRead,ok,log,f;
    pid_t pid,pidd;
    pid=fork(); f=open("alice.txt",O_RDWR|O_TRUNC|O_CREAT,0700);

    if(pid == 0){
        i=0; 
        printf("f: %d",f);
        if(strlen(input) == 2) {arg[0]="ls";arg[1]=NULL; i=2;}
        else 
        {       
            result=strtok(input,delim);
            arg[i++]=result;
            result=strtok(NULL,delim);
            while(result!= NULL)
            {
                printf("LS --- 5\n");
                arg[i++]=result;
                result=strtok(NULL,delim);
            }
            arg[i]=NULL;
        }

        close (1);

        if (dup2(f,1) == -1)    
        {
            fprintf (stderr, "dup - 1\n");
            exit (9);
        }

        if( 0 == (pidd=fork())) { 
            execvp("ls",arg);
        }

        close(f);
        i=0;
        while(0 != read(1,&ch,sizeof(char)))
        { 
            aux4[i]=ch;
            i++;
        }
        aux4[i]='\0';

        close(1);
        if (dup2(p2[1],1) == -1)    
        {
            fprintf (stderr, "dup - 1\n");
            exit (9);
        }
        //close(p2[1]);
        len=strlen(aux4);
        //printf("LUNG: %d",len);
        write(1,&len, sizeof(4));       
        return 0;
    } else {
        wait(NULL);
        close(p2[1]);
        read(p2[0],&len,sizeof(int));
        printf("pp: %d",len);

    }
}

Answer 1

Instead of forking and duping yourself you can conveniently use popen to do what you want. If you really need to know the size of the output beforehand, you don't need to redirect to a file, you can simply count the bytes you can read from the pipe before you hit EOF. Example (no error checking):

#include <stdio.h>
#include <stdlib.h>

int main()
{
    FILE *fd;
    char *buf;
    size_t size; /* counter for output size */

    fd = popen("ls", "r"); /* open a pipe running ls for reading ("r") */

    size = 0;
    while (fgetc(fd) != EOF) {
        size++;
    }

    pclose(fd);

    printf("Output is %d bytes\n", size);

    fd = popen("ls", "r");

    buf = malloc(size+1);

    while (fgets(buf, size, fd)) {
        fputs(buf, stdout); /* do what you want here */
    }

    pclose(fd);

    return 0;
}

Note that if you intend to parse the ls output later to find files or directories, you are much better off using the opendir, readdir and closedir functions on Unix.